Monotremes. Characterized by the
duckbilled platypus and the spiny anteaters, lay eggs and
maintain some reptilian characteristics. They do not contain
true mammary glands, but produce a fatty sweat (milk) from
glands in the skin. The milk collects and drips down tufts of
hair into the offspring's mouth. They are found in Australia
and New Guinea.

Marsupials. These mammals contain a
pouch (marsupium).
Opossums,koalas,kangaroos, and other examples live in Australia
as a result of the break up of the super continent
Pangea. The young are born during
the early stages of development. The new born crawls up to the
mother's pouch, where it clings on to a nipple and hangs there
until it fully develops.

Placental
Mammals.
These mammals are the most abundant and diverse of the class.
The placenta, a reproductive structure, is housed in the uterus
of the female. Here the offspring receives all that it needs to
develop into a fully developed organism. This period of
development (gestation) varies with the species of mammal.
Whales gestate for 2 years, while a mouse develops in 21
days.

Chart of Placental Mammals

Order

Characteristics

Examples

Carnivora

Eats meat, all teeth are
pointed

cats and dogs , seals, walrus,
weasels, and martins

Chiroptera

wings used for flight, diet includes
insects and vegetation

bats

Primates

opposable thumb, plantigrade foot,
usually one offspring at birth.

man , apes, monkeys,tarsiers,and
lemurs

Edentata

lacking teeth, large claws for
digging

sloths, armadillos, South
American anteater

Rodentia

2 incisors on both jaws, diastema
between incisors and premolars

rats, mice, squirrels, beavers,
gophers, and capybara

Lagmorpha

4 incisor teeth, canine teeth
lacking, short stubby tails

rabbits,
hares,pikas

Cetacea

flippers, opening on top of
head

dolphins,whales,porpoises

Proboscidea

Great size,nose and upper lip
form proboscis, upper incisors are tusks,thick skin,scant
hair